1. Field of the Invention
This invention relates to a semiconductor laser which emits laser light by injecting current, and more particularly relates to a ridge waveguide-type semiconductor laser having a current recombination layer (an active layer) on which a belt-like projection extending in the direction of laser resonance is formed.
2. Description of the Prior Art
Heretofore, mesa-etching processing for forming a ridge waveguide of a ridge waveguide-type laser has usually been performed, as dislosed in U.S. Pat. No. 4,142,160, for example, by removing semiconductor layers laminated on an active layer to a depth just reaching or adjacent to the active layer.
In such a conventional process, when the etched depth is relatively shallow, and the distance X.sub.R between the upper surface of the etched regions at the both sides of the ridge and the upper surface of the active layer is no less than 0.2 .mu.m, a gain guide-type semiconductor laser is obtained, since the difference in effective refractive index between the ridge waveguide region and the exterior thereof is small. Consequently, the laser has characteristics such that the threshold current is relatively large, the spread angle (.theta..parallel.) of the far field pattern (FFP) in the horizontal direction is small, astigmatism (AS) is large or the like. Now, when X.sub.R is no more than 0.2 .mu.m by increasing the etching depth, a laser having a large refractive index guide effect is obtained, and thus the spread angle in the horizontal direction and astigmatism are improved.
When the etching depth is increased as described above, however, damage caused by the processing reaches the active layer. Hence, an increase of threshold current, a decrease of quantum efficiency, a decrease of the device life or the like occur. There is also a problem that the variance .DELTA.X due to the processing accuracy of the above-described X.sub.R largely influences several characteristics of the laser. This is because the change which .DELTA.X causes in the effective refractive index is larger in the region where X.sub.R is small than in the region X.sub.R is large. That is, in order to produce a ridge waveguide-type laser having a large refractive index guide effect, a high processing accuracy of .DELTA.X&lt;0.1 .mu.m is needed at X.sub.R .perspectiveto.0.1-0.2 .mu.m, and so production is difficult.